I thought I would be able to find this out quickly. I went to look up explosion's z move base power, and it turns out it actually gets powered down from 250 base power to 200 base power when it gets turned into breakneck blitz (probably because when you use it as a z move, you don't also faint). And OHKO moves like fissure all have a z move base power of 180. And moves with varying base power all have a z move base power taken from their minimum base power, regardless of what it's normal base power would actually be in any circumstance.

So what is the highest base power z move? And if it's an exclusive one, then also what is the highest base power non-exclusive one?